Cold-cathode gas-discharge tubes



A ril 21, 1959 D. w. HILL 2,883,584

COLD-CATHODE GAS-DISCHARGE TUBES Filed March 8, 1956 A/VEA/TGP 2,883,584 Patented Apr. 21, 1959 COLD-CATHODE GAS-DISCHARGE TUBES Dennis Walter Hill, New Southgate, London, England, assignor to Hivac Limited, South Ruislip, England, a British company Application March 8, 1956, Serial No. 570,367

Claims priority, application Great Britain March 17, 1955 18 Claims. (Cl. 315-335) This invention relates to cold-cathode gas-discharge tubes and is more particularly, although not exclusively concerned with tubes of this type suitable for speech transmission purposes.

In connection with such cold-cathode gas-discharge tubes it is already known that, when a DC. potential suitable for maintaining a discharge is applied to the electrodes, the current through the tube includes a number of spurious variations or noise components and although some of these can be suppressed or prevented by the adjustment or design of the external circuit associated with the tube, other components cannot thus be eliminated, and the utility of the tube for certain purposes, such as speech transmission, is adversely affected. A particularly disadvantageous phenomena is the generation of oscillatory noise components especially when these have a frequency lying near to or within the audio frequency range.

With a view to eliminating these oscillatory noise" signals it has already been proposed to construct a coldcathode gas-discharge tube in which the anode is so arranged that it is located in the Faraday dark space of the anode-cathode discharge at a position where anode glow does not occur. While advantageous from the aspect of avoiding generation of oscillatory noise voltages, such a construction has the efi'ect of materially reducing the voltage differential between the striking and maintaining voltages for the discharge and this is extremely disadvantageous in most circuit applications inasmuch as it materially restricts the freedom of circuit design.

Gas-discharge tubes of the type with which the present invention is concerned, when used for speech transmission and certain other purposes, desirably have a low value of positive dynamic resistance or even, under some circumstances, a negative dynamic resistance. Certain tube constructions already proposed can provide such a low positive or even negative value of dynamic resistance but hitherto such low positive or negative'values have occurred only at relatively high values of the discharge current, say, 10 Ina. or above. Such high current values are themselves frequently disadvantageous in again setting undesirable limits on the parameters of the associated circuits and also of causing a relatively heavy current drain particularly when a considerable number of the tubes are in use simultaneously as may be the case in the application of such tubes to telephone switching circuits and the like.

Among the objects of the present invention is that of providing a tube which has a low value of self-generated oscillatory noise voltages while still possessing a clearly defined and appreciable voltage diiferential between the initial striking and the maintaining voltages for the discharge and also that of providing a coldcathode gasdischarge tube construction which is capable of providing a low positive or even negative value of dynamic resistance accompanied by a relatively low value of main discharge current.

In accordance with the invention a cold-cathode gasdischarge tube comprises an anode in the form of a rod sunk Within a recess in a body of insulating material with the ratio of recess diameter to anode diameter lying within the range of 5:1 and 2:1 and a cathode arranged for co-operation with said anode, said cathode being positioned opposite the open end of said recess at such a spacing distance from the anode that, with the particular gaseous filling and gas pressure employed, the anode is positioned just outside the Faraday dark space of the anode-to-cathode discharge whereby the self-generated oscillatory noise voltages are of low value and have a central frequency substantially above the audio frequency range.

The term low value used above with relation to the self-generated oscillatory noise voltages is to be construed as meaning voltage values which are at least 40 db below the mean signal voltages which the tube is capable of handling under normal operating conditions.

The invention is applicable to diode structures but is more particularly advantageous in connection with threeelectrode trigger tube devices in which the initiation of the discharge between the cathode and anode is effected by means of a suitable control voltage applied to a third or trigger electrode. In the construction of such a trigger tube according to the invention the cathode conveniently comprises a part providing an end wall surface facing the open end of said recess with the trigger electrode located in the space between said cathode surface and said open end of the recess. The gas filling employed is preferably of pure helium or a mixture of gases embodying a major proportion of helium while the gas pressure used is preferably one which is in excess of 50 mm. of mercury.

In order that the above and other features of the invention may be more readily understood it will now be described in greater detail with reference to the accompanying drawing, in which:

Fig. 1 is a longitudinal axial section through a discharge tube embodying the present invention.

Fig. 2 is an end view and Fig. 3 is a side elevational View of a sub-assembly of the various electrode elements before enclosure within the surrounding envelope.

Fig. 4 is a diagram drawn to an enlarged scale for the purpose of illustrating a number of relatively critical dimensions of the embodiment of the tube shown in Fig. 1.

Fig. 5 is a perspective view showing an alternative form of cathode construction to that shown in Fig. 1, while Fig. 6 is a circuit diagram showing one practical application of the tube for speech transmission purposes.

In the embodiment shown in Figs. 1, 2 and 3 the tubular glass envelope 10 enclosing the various electrodes has a flattened pinch region 11 at one end and contains a suitable gaseous filling as referred to later.

The tube electrodes comprise an anode in the form of a wire or rod 12 which is tipped with nickel at its upper end and which is disposed substantially coincident with the axis of the envelope. The lower end of this wire extends outwardly through the pinch region 11 to provide an external connecting lead. The anode wire 12 is enclosed within a co-axial tubular recess 13 formed at the centre of a glass bead 14. The recess 13 may be very slightly flared outwardly as shown. The glass head 14 is shaped as a disc and is formed integrally with a tubular glass stem 15 the bore of which constitutes a continuation of the aforesaid recess 13. This stem, although initially formed as a separate element as shown in Fig. 2 is eventually united at its closed lower end with the pinch region 11 of the envelope.

The cathode, indicated generally at 17, comprises a thin metal disc 18 disposed opposite the open end of the recess 13 in a plane normal to the axis of the anode wire 12 and co-axial therewith. This disc is of molybdenum and is provided with a small diameter central aperture 24 to permit observation of the discharge. A cylindrical skirt 19, of thin sheet nickel, is provided around the peripheral edge of the disc 18 and is secured to the latter by means of bent-over lugs 20 which are united with the disc 18 by spot welding. The cathode 1'7 is located as shown with the skirt l9 directed towards the anode and having its lower marginal edge close to but not touching the peripheral edge surface of the bead 14. The cathode is secured by means of two support wires 21 which are spot welded at one end to diametrically opposed points of the outer surface of the skirt and connected respec tively at their lower ends to bent wire lugs 22 embedded in the glass bead 14. in this way the discharge space between the anode and the cathode is substantially enclosed and is effectively screened from any electrostatic charges which may be acquired by the inner wall of the tube envelope 10. A wire 23, joined to one of the wires 21, passes outwardly through the pinch 11 as an external connection lead.

The skirt 19 is not quite a complete cylinder, there being a gap as shown at 25 to accommodate the extension arm 2% of a trigger electrode 27 which is in the form of a circular wire loop disposed co-axial with the anode rod 12 in a plane parallel to that of the cathode disc 10. The extension arm 28 is secured at its outer end to a further bent wire support lug 29 also embedded in the bead 14. This extension arm 25 is also electrically connected to a lead-out wire 30 which passes through the pinch region 13. Preferably that region of the extension arm 28 which lies immediately adjacent the gap 25 in the cathode skirt 19 is suitably treated, as by calorising, to cause its surface to have a high work function and thereby to inhibit any discharge taking place thereto.

A gettering element 32 is provided below the head 14 and is supported in position by means of a further wire lug 33 also embedded in the bead.

The positioning of the nickel-tipped active end of the anode wire 12 with relation to the inner active surface of the cathode disc 18 is important and relatively critical since the anode must lie outside, but only just outside, the Faraday dark space formed when discharge takes place between the anode and cathode. Such positioning is obviously dependent upon the nature of the gaseous filling employed and the pressure of such filling. In one particular embodiment constructed in accordance with Fig. 1 and constituting a gas-discharge trigger tube in the subminiature class, the various critical dimensions were as follows, referring to Fig. 4. With a gaseous filling consisting of a mixture of helium 76%, neon 21% and argon 3% (all by volume) and a gas pressure of 70 mm. mercury, the diameter A of the open end of the recess 13 was 1.5 mm., the diameter B of the anode rod 12 was 0.4 mm, the spacing distance C of the active end surface of the anode rod 12 below the level of the open end of the recess 13 was between 0.3 and 0.4 mm. while the separation distance D measured along a projection of the axis of the anode rod, between the active end of such rod and the plane of the cathode surface 18, was between 2.25 and 2.45 mm. The spacing distance E between the plane of the trigger electrode 27 and the cathode surface 18 was between 1.15 and 1.35 mm.

Such tube had a main cathodeto-anode discharge current of 2 ma. DC. and with this relatively low value of discharge current it was found possible to obtain a value of dynamic resistance which was variable between +700 ohms and -700 ohms by altering the trigger electrode current from approximately 700 ,ua. to approximately 70 p.21. Such variation of the trigger current value and, hence, control of the impedance value of the tube, was conveniently effected by means of series resistances of between 62 kiloohms and l megohm inserted between the trigger electrode and its (assumed constant) operating potential source of +150 v.

The initial striking voltage of the main discharge between cathode and anode was 250 v. and the maintaining voltage v. giving a differential voltage of some volts. Its discharge current was of the order of 2 ma. D.C. Such a tube was found capable of transmitting 0.3 ma. R.M.S. of alternating current at a frequency of 1,000 cycles per second and had a value of self-generated oscillatory noise which was more than 50 db down on the input alternating current signal. The noise voltages were found to have a central frequency of around about 250 kc./ s. It was found satisfactory for transmitting audiofrequency signals in the frequency band 300-3000 c.p.s.

An alternative form of cathode for such a. tube is shown in Fig. 5 at 17a where the aperture 24 of the earlier embodiment is dispensed with and the disc 18a formed as a complete disc of somewhat smaller diameter than the inner diameter of the surrounding skirt 19 whereby an annular aperture 24:: is provided for observing the discharge. The skirt 19 is provided, as before, with inturned lugs 20 suitably extended for connection to the disc 18a.

A variety of cathode shapes may be employed including a flat disc, a part-hemisphere or even a substantially complete hemisphere practically enclosing the discharge space but the particular shapings illustrated in Figs. 1-3 or 5 have so far been found the most advantageous. Although molybdenum is preferrcd for forming the disc 18 in order to provide a staole cathode surface, other metals such as Zirconium may be used,

The trigger electrode, although preferably of ring formation and having a diameter comparable with that of the anode recess, may be of a shape other than that shown in Fig. 1. For example, it may consist of a flat annulus disposed parallel with the surface of the disc 18.

The gaseous filling employed may consist of a mixture of inert gases different from that already described but it is preferable always to use a major proportion of helium. Alternatively pure helium may be used. The use of helium or a gaseous mixture consisting mainly of helium provides a wider tolerance of the spacing distance by which the anode can be located outside the Faraday dark space. While the use of other gases is possible, the positioning of the anode then becomes much more critical. It is also desirable to employ a relatively high gas-filling pressure, in excess of 50 mm. of mercury, in order to produce a compact discharge.

Similarly the relative dimensions given for the anode electrode and its surrounding recess may be varied considerably with, of course, corresponding effect upon the relative positioning of the cathode and upon the gaseous filling and filling pressure necessary. The preferred ratios of the dimension A to dimension B of Fig. 4 are between 7:2 and 2:1 while the ratio of the dimension B to the dimension C should be between 4:3 and 2:5. Undue increase of the dimension C tends to increase the negative dynamic resistance of the resultant tube.

A particular practical application of a tube according to the invention is to speech current transmission in telephony and other communication circuits. Fig. 6 illustrates one such application where the tube GT is disposed in a circuit connection between an input transformer Ti and an output transformer T0. The cathode 17 is connected by way of the secondary winding of transformer Ti and resistance R1 to a source of negative potential while the anode 12 is connected by way of the primary winding of transformer T0 and resistance R2 to a source of positive potential. The trigger electrode 27 is connected by way of resistance R3 to a source of control potential which is of positive polarity with respect to the cathode 17. Condensers C serve to provide bypass paths for A.C. around the resistances R1, R2 and the potential sources.

Initiation of the discharge within the tube to connect the transformers Ti, To may be effected in several ways such as by the application of a suitably increased positive potential to the trigger electrode 27. When operative, the impedance of the tube GT may be varied by suitable control of the value of current flowing to the trigger electrode 27. This may be elfected by making the resistance R3 Variable as shown. In this case the value of the operating potential for the trigger electrode may be chosen to be above the breakdown voltage of the trigger electrode-cathode gap whereby the tube automatically restrikes immediately should the anode/ cathode gap discharge be extinguished by the passage of a large amplitude signal pulse. In an alternative arrangement the resistance R3 is of fixed value and the necessary adjustment of the value of trigger electrode current is effected by altering the value of the potential source for such trigger electrode.

The construction of the tube in sub-miniature sizes is particularly advantageous for telephonic and other communication purposes where small size is essential while the ability of the tube to provide a negative dynamic resistance is highly advantageous in effecting reduction of circuit losses. The tube is also adapted for use as a voltage stabiliser or for the purpose of increasing the Q value of an LC filter circuit.

I claim:

1. A cold-cathode gas-discharge tube comprising an envelope containing a low-pressure gaseous filling and an electrode structure within said envelope, said electrode structure including a body of insulating material defining a tubular recess having an open end, an anode rod disposed wholly within said recess with its end sunk below said open end of said recess and a cathode plate disposed in a position directly facing said open end of said recess, the spacing distance between said cathode plate and said end of said anode rod being such that, with the gaseous filling and gas pressure within said envelope, said anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge when the tube is energised.

2. A cold-cathode gas-discharge tube comprising an envelope containing a low-pressure filling of helium gas and an electrode structure within said envelope, said electrode structure including a body of insulating material defining a tubular recess of circular cross-section and having an open end, an anode rod of circular crosssection disposed wholly within said recess with its axis coaxial with that of the recess and with its end sunk below said open end of said recess and a cathode plate disposed in a position directly facing said open end of said recess, the diameter of said recess being between two and five times that of said rod and the spacing distance between said cathode plate and said end of said anode rod being such that, with the gaseous filling and gas pressure within said envelope, said anode is positioned just outside the Faraday dark space of the cathodeto-anode discharge when the tube is energised.

3. A cold-cathode gas-discharge tube comprising an envelope containing a low-pressure gaseous filling, an electrode structure within said envelope, said electrode structure comprising a body of insulating material having a circular section tubular recess therein, said recess having an open end and one closed end, an anode rod of circular section disposed coaxially within said recess and extending from said closed end towards said open end and with the free end of said anode rod sunk below the level of said open end of said recess by a distance which is between two and a half times and three-fourths the diameter of said anode rod, a plate-like cathode element disposed directly opposite to and spaced away from said open end of said recess in a plane normal to the axis of said anode rod, said spacing distance between said cathode and the nearest end of the anode rod being such that, with the particular gaseous filling and gas pressure employed, said anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge when the tube is energised.

4. A cold-cathode gas-discharge tube comprising an envelope containing a low-pressure gaseous fillingconsisting predominantly of helium, an electrode structure with said envelope, said electrode structure comprising a body of insulating material having a tubular recess therein, said recess having one open end and one closed end, an anode rod disposed coaxially within said recess and extending from said closed end towards said open end and with the free end of said anode rod sunk below the level of said open end of said recess, a plate-like cathode element disposed opposite and spaced away from said open end of said recess in a plane normal to the axis of said anode rod, said spacing distance between said cathode and the nearest end of the anode rod being such that, with the particular gaseous filling and gas pressure employed, said anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge when the tube is energised.

5. A cold-cathode gas-discharge tube comprising an envelope containing a low-pressure gaseous filling, an electrode structure within said envelope, said electrode structure comprising a body of insulating material having a tubular recess therein, said recess having one open end and one closed end, an anode rod disposed within said recess and coaxial therewith and extending from said closed end towards said open end, said anode rod having a free end facing towards said open end of said recess but disposed at a level which is sunk within said recess, the ratio of the recess diameter to the diameter of said anode rod lying within the range of 5:1 and 2:1 and the ratio of the distance by which the end of the anode rod is sunk within said recess to the diameter of said rod lying within the range of 5:2 and 3:4 and a plate-like cathode for cooperation with said anode, said cathode being positioned facing said open end of said recess in a plane normal to the axis of said anode rod and at a spacing distance from the free end of said anode rod such that, with the particular gaseous filling and gas pressure employed, said anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge.

6. A cold-cathode gas-discharge tube comprising an envelope containing a low-pressure gaseous filling, an electrode structure within said envelope, said electrode structure comprising a body of insulating material having a tubular recess therein, said recess having one open end and one closed end, an anode rod disposed within said recess and coaxial therewith, said anode rod having a free end facing towards said open end of said recess but disposed at a level which is sunk within said recess, the ratio of the recess diameter to the diameter of said anode rod lying within the range of 5:1 and 2:1 and the ratio of the distance by which the end of the anode rod is sunk within said recess to the diameter of said rod lying within the range of 5 :2 and 3:4, a plate-like cathode for co-operation with said anode, said cathode being positioned facing said open end of said recess in a plane normal to the axis of said anode rod and at a spacing distance from the free end of said anode rod such that, with the particular gaseous filling and gas pressure employed, said anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge and a ring-shaped trigger electrode located between said open end of said recess and said cathode.

7. A cold-cathode gas-discharge tube comprising an envelope, a low-pressure gaseous filling within said envelope, an electrode structure within said envelope, said electrode structure comprising a body of insulating material having a tubular recess therein, said recess having one open end and one closed end, an anode rod located in said recess and coaxial therewith, said anode rod having a free end which is sunk below the level of said open end of said recess, a cathode having a plate-like portion disposed in a position facing said open end of said recess and in a plane normal to the axis of said anode rod and a skirt portion on said cathode extending from the periphery of said plate-like portion towards said body of insulating material to enclose the space in which the cathodeto-anode discharge takes place, the spacing distance between the centre of said plate-like portion of said cathode and said free end of said anode rod being such that, with the particular gaseous filling and gas pressure employed, the anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge when the tube is energised.

8. A cold-cathode gas-discharge tube comprising an envelope, a low-pressure gaseous filling within said envelope, an electrode structure within said envelope, said electrode structure comprising a body of insulating material having a tubular recess of circular cross section therein, said-recess having an open end and a closed end, an anode rod located in said recess and coaxial therewith, said anode rod having a free end which is sunk below the level of said open end of said recess, a cathode having a plate-like portion disposed in a position facing said open end of said recess and in a plane normal to the axis of said anode rod and a skirt portion of said cathode extending from the periphery of said plate-like portion towards said body of insulating material to enclose the space in which the cathode-to-anode discharge takes place, the diameter of said recess being between two and five times that of said rod and the spacing distance between the centre of said plate-like portion of said cathode and said free end of said anode rod being such that, with the particular gaseous filling and gas pressure employed, the anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge when the tube is energised.

9. A cold-cathode gas-discharge tube comprising an envelope, an electrode structure within said envelope, a low-pressure gaseous filling of helium in said envelope, said electrode structure comprising a body of insulating material shaped as a bead and having a central tubular recess therein, said recess having an upper open end and a closed lower end, an anode rod positioned in said recess, said anode rod having a free end which faces but is sunk below the level of said open end of said recess, a cathode co-operating with said anode, said cathode having a plate-like portion which faces said open end of said recess and the free end of said anode rod and which is disposed in a plane substantially normal to the axis of said anode rod and a skirt-like portion which extends from the periphery of said plate-like portion to the vicinity of the outer edge of said bead so as to enclose the space in which the cathode-to-anode discharge takes place and a trigger electrode disposed in between said open end of said recess and said cathode, the axial sp ac ing distance between said cathode portion and said free end of said anode rod being such that, with the particular gaseous filling and gas pressure employed, the anode is positioned just outside the Faraday dark space of the cathode-to-anode discharge when the tube is energised.

10. A cold-cathode gas-discharge tube comprising an envelope, a low-pressure gaseous filling in said envelope, an electrode structure within said envelope, said electrode structure comprising a body of insulating material having a circular section tubular recess therein, said recess having one open end and one closed end, an anode rod positioned in said recess to lie coaxial therewith, said anode rod having a diameter which is between one half and one fifth of the diameter of said recess and having a free end which faces but is sunk below the level of said open end of said recess, -a cathode co-operating with said anode, said cathode having a plate-like portion directly opposite to and facing said open end of said recess and the free end of saidanode rod and being disposed in a plane substantially normal to the axis of said anode iI'Od and a trigger-electrode disposed in between the open end of said recess and said cathode, the axial spacing distance between said cathode portion and said free end of said anode rod being such that, with the particular gaseous filling and gas pressure employed, the anode is positioned just outsidethe Faraday dark space of the cathode-to-anode discharge when the tube is energised.

11. A gas-discharge tube according to claim 10 in which the gaseous filling is of helium.

12. A gas-discharge tube according to claim 10 in which the gaseous filling comprises a mixture, the major part of which is of helium.

13. A gas-discharge tube according to claim 10 in which the ratio of the distance by which the free end of said anode rod is sunk within said recess to the diameter of said anode rod lies within the range of 5:2 and 3:4.

14. A gas-discharge tube according to claim 10 in which the cathode is shaped and positioned so as substantially to enclose the space in which the anode/ cathode discharge takes place.

15. A gas-discharge tube according to claim 14 in which the cathode comprises a cup-shaped elementhaving a fiat disc-shaped end wall forming said plate-like portion and a skirt around the periphery of said disc, said skirt being directed towards the anode.

16. A gas-discharge tube according to claim 15 in which said recess is formed in a body of insulating material shaped as a beadlike member which substantially closes the open end of said skirt.

17. A signal transmission circuit including within its signal transmission path a cold-cathode gas-discharging tube according to claim 10 and comprising also a source of current for said trigger electrode and circuit means connecting said source to said trigger electrode, said circuit means including means for varying the value of current flow to said trigger electrode for adjusting the dynamic impedance of said tube.

18. A signal transmission circuit including within its signal transmission path a cold-cathode gas-discharge tube comprising an envelope, a gaseous filling in said envelope and an electrode structure within said envelope, said electrode structure including a body of insulating material having a tubular recess therein, said recess having an open end, an anode rod disposed within said recess with its end sunk below the level of said open end of said recess, a plate-like cathode member opposite said open end of said recess, said cathode member being spaced from the free end of said anode rod by a distance which, with the particular gaseous filling and gas pressure employed, is just outside the Faraday dalrk space of the cathode-to-anode discharge and a trigger electrode disposed in between the open end of said recess and said cathode, a source of current for said trigger electrode and circuit means connecting said source to said trigger electrode, said circuit means including means for varying the value of current flow to said trigger electrode for adjusting the dynamic impedance of said tube.

References Cited in the file of this patent UNITED STATES PATENTS 1,939,063 Knowles Dec. 12, 1933 2,062,268 Knowles Nov. 24, 1936 2,106,847 Kniepkamp Feb. 1, 1938 2,340,799 Depp Feb. 1, 1944 2,349,012 Spaeth May 16, 1944 

